Chemical Engineering Journal 228 (2013) 700–707

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Chemical Engineering Journal

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Simultaneous absorption of SO2 and NOx with pyrolusite slurrycombined with gas-phase oxidation of NO using ozone: Effectof molar ratio of O2/(SO2 + 0.5NOx) in flue gas

1385-8947/$ - see front matter � 2013 Elsevier B.V. All rights reserved.http://dx.doi.org/10.1016/j.cej.2013.05.003

⇑ Corresponding author. Tel./fax: +86 28 8546 0916.E-mail address: ssjscu@163.com (S.-j. Su).

Sun Wei-yi, Wang Qing-yuan, Ding Sang-lan, Su Shi-jun ⇑College of Architecture and Environment, Sichuan University, Chengdu 610065, China

h i g h l i g h t s

� Competitive oxidation of SO2/NOx by MnO2 and O2 in absorption process.� Increasing O2/(SO2+0.5NOx) enhanced catalyzed oxidation between O2 and SO2/NOx.� Total absorption capability of SO2 and NOx increased with O2/(SO2+0.5NOx).� Mn extraction rate increase first and then decrease with O2/(SO2+0.5NOx).� Mn extraction rate can be enhanced by adding rhodochrosite as pH regulator.

a r t i c l e i n f o

Article history:Received 13 December 2012Received in revised form 20 March 2013Accepted 4 May 2013Available online 20 May 2013

Keywords:Molar ratio of O2/(SO2 + 0.5NOx)Catalyzed oxidationByproductsMn extraction rateAbsorption capability

a b s t r a c t

NO in flue gas was oxidized into NO2 by ozone in gas-phase first, and then SO2 and NO2 were oxidized byredox reaction between MnO2 and SO2/NO2 and catalyzed oxidation between O2 and SO2/NO2 in absorp-tion process. Molar ratio of O2/(SO2 + 0.5NOx) in flue gas had a decisive effect on catalyzed oxidation inabsorption process, and eventually influenced reaction byproducts, Mn extraction rate and SO2/NOx

absorption capability. Results showed that increasing O2/(SO2 + 0.5NOx) enhanced catalyzed oxidationand led to the lower reaction pH. When O2/(SO2 + 0.5NOx) 6 18, SO2 and NOx were mainly oxidized byMnO2 with main products of MnSO4 and Mn(NO3)2, while when O2/(SO2 + 0.5NOx) P 18, SO2 and NOx

were mainly oxidized by O2 with main products of H2SO4 and HNO3. Total absorption capability of SO2

and NOx increased with increasing O2/(SO2 + 0.5NOx). Mn extraction rate increased with the increase ofO2/(SO2 + 0.5NOx) first and then decreased, the maximum Mn extraction rate of 91% was got when O2/(SO2 + 0.5NOx) at around 13. Both the SO2/NOx absorption capability and Mn extraction rate could beenhanced by adding rhodochrosite as pH regulator in the case of O2/(SO2 + 0.5NOx) P 18.

� 2013 Elsevier B.V. All rights reserved.

1. Introduction

NOx(NO and NO2) and SO2 emission from combustion of coal,fuel oils and waste have given significant effects on environment[1,2] and human health [3–5]. Various kinds of technologies havebeen developed to control and reduce SO2 and NOx emissionsworldwide. Wet absorption processes are widely used for SO2 re-moval due to advantages of high SO2 removal efficiency, simpleconstruction, low operating cost and good reliability. But it is diffi-cult to attain high NOx removal efficiency at the same time becauseof the low solubility of NO which accounts for more than 90% of allNOx [6–8]. A large number of liquid oxidants with strong oxidabil-ity, such as sodium chlorite [9–14], hydrogen peroxide [15–18] andpotassium permanganate [19–22], are used as absorbent to obtain

the satisfying NOx removal efficiency as SO2 in wet process.Although using strong oxidizing agents can improve NOx removalefficiency, there are certain drawbacks, such as the expensive costof oxidizing agents or disposal problems of absorption solution[23].

Manganese is an important metal in human life and industry. Inrecent years, rapid growing demand for manganese products hasdramatically leaded to the decreases of manganese ores both inquantity of and grade. Low grade manganese oxide ores(Mn% < 40%) become increasingly important sources of manganese[24]. However, these manganese oxide ores were not economic inmetallurgy because Mn exists in the form of MnO2 which can notreact with H2SO4 and will be reduced into MnO first by carbon atabout 700–900 �C [25], leading to complex system, high invest-ment, operating cost and emissions of greatest amount of CO2. Inrecent years, various direct reductive leaching processes have beenstudied to recovery Mn from these low grade manganese oxide

W.-y. Sun et al. / Chemical Engineering Journal 228 (2013) 700–707 701

ores [26–33], among which SO2 and NO2 have proved promisingattributing to advantages of fast leaching rate, high Mn extractionrate and selectivity leaching of Mn [28–33].

To combine the removal process of SO2 and NOx from flue gaswith the reductive leaching processes of manganese oxide ores,low grade pyrolusite, one of the manganese oxide ores, was usedas absorbent to remove SO2 and NOx from flue gas. SO2 and NOx

in flue gas were exploited as cost effective reductants to react withMnO2 in pyrolusite to create MnSO4 and Mn(NO3)2 which arechemical industry products. In order to get a higher NOx removalefficiency, O3 was used as gaseous oxidant to oxidize NO to higherorder nitrogen species (NO2, NO3 and N2O5) due to its advantagesof selective oxidation of NO, high oxidation efficiency, fast oxida-tion speed and non-pollution decomposition products [34–40]. Itis undeniable that N2O5 is much higher soluble and more easiercaptured than NO2 by wet scrubbers, however, when pyrolusitewas used as absorbent, oxidization of NO into NO2 would be moreadaptable, because N2O5 can react with water to produce HNO3,while NO2 react with MnO2 to produce Mn(NO3)2. In addition, oxi-dization of NO into NO2 need less O3 injection and shorter reactiontime [34], this will reduce the cost of pro-oxidization of NO.

Our previous studies showed that in the absorption process, ex-cept for MnO2 in pyrolusite, the dissolved oxygen can also oxidizedSO2 and NOx into H2SO4 and HNO3 in the presence of Mn2+ whichwas produced as the reduction product of MnO2 [41]. The concen-trations of O2, SO2 and NOx, which changed in an actual flue gaswith fire conditions and contents of S and N in fuel, may haveimportant effect on oxidation rates between O2 and SO2/NOx, andultimately affect Mn extraction rate and SO2/NOx absorption effi-ciency, which are two important parameters for SO2 and NOx re-moval process with pyrolusite slurry. In this paper, the effect ofthe relationship between the concentrations of O2, SO2 and NOx

on the absorption process was theoretically analyzed. The O2/(SO2 + 0.5NOx) (molar ratio) in the flue gas was put forward as acorrelated parameter, and its effect on catalyzed oxidation,byproducts, Mn extraction rate, SO2/NOx absorption efficiencyand absorption capacity, were investigated, and the measures forincreasing SO2/NOx absorption capability and Mn extraction ratewere presented. The objective of the research was to provide thetheoretical basis for obtaining satisfied SO2/NOx absorption capa-bility and Mn extraction rate for flue gas with differentcomponents.

2. Theoretical

The simultaneous removal system of NOx and SO2 consisted oftwo processes: (1) pro-oxidation of NO into NO2 using ozone ingas phase, (2) absorption of NO2 and SO2 with pyrolusite slurryin liquid phase.

In the pro-oxidation process, when ozone was injected to fluegas, NO can be oxidized into NO2 rapidly (R1) and then the excessozone can further oxidize NO2 into NO3 and N2O5 (R2 and R3).When the input ozone concentration is less than NO concentration,NO2 was the predominant oxidation product [34–41]. Ozone hadlittle impact on SO2 oxidation, because the activation energy ofSO2 reaction with ozone was 3.5 times as high as NO with ozone,and rate constant of the former was 1/105 of the latter [40].

NOðgÞ þ O3ðgÞ ¼ NO2ðgÞ þ O2ðgÞ ð1Þ

NO2ðgÞ þ O3ðgÞ ¼ NO3ðgÞ þ O2ðgÞ ð2Þ

NO2ðgÞ þ NO3ðgÞ ¼ N2O5ðgÞ ð3Þ

In absorption process, O2, SO2, NO2 as well as very little NO3 andN2O5 in oxidized flue gas dissolve into the liquid phase first

(R4–R12) and then the low order conversions of SO2 and NO2 wereoxidized into SO2�

4 and NO�3 by MnO2 and O2 (R13–R22).Reactions occur during dissolution process

O2 ¼ O2ðlÞ ð4Þ

NO2 ¼ NO2ðlÞ ð5Þ

2NO2ðlÞ þH2O ¼ NO�3 þ NO�2 þ 2Hþ ð6Þ

NO2ðgÞ þ NO3ðgÞ þH2O ¼ 2NO�3 þ 2Hþ ð7Þ

N2O5ðgÞ þH2O ¼ 2NO�3 þ 2Hþ ð8Þ

3HNO2ðlÞ ¼ Hþ þ NO�3 þ 2NOþH2O ð9Þ

SO2 ¼ SO2ðlÞ ð10Þ

SO2ðlÞ þH2O ¼ Hþ þHSO�3 ð11Þ

HSO�3 ¼ Hþ þ SO2�3 ð12Þ

Oxidations of NO2/SO2 by MnO2 in liquid phase:

Hþ þMnO2ðsÞ þHNO2ðlÞ ¼Mn2þ þ NO�3 þH2O ð13Þ

MnO2ðsÞ þ 2NO2ðlÞ ¼ Mn2þ þ 2NO�3 ð14Þ

MnO2ðsÞ þ SO2ðlÞ ¼Mn2þ þ SO2�4 ð15Þ

MnO2ðsÞ þHþ þHSO�3 ¼Mn2þ þ SO2�4 þH2O ð16Þ

MnO2ðsÞ þ 2Hþ þ SO2�3 ¼Mn2þ þ SO2�

4 þH2O ð17Þ

Oxidations of NO2/SO2 by O2 in liquid phase:

SO2ðlÞ þ 1=2O2ðlÞ þH2O ¼ 2Hþ þ SO2�4 ð18Þ

1=2O2ðlÞ þHSO�3 ¼ Hþ þ SO2�4 ð19Þ

1=2O2ðlÞ þ SO2�3 ¼ SO2�

4 ð20Þ

2NO2ðlÞ þ 1=2O2ðlÞ þH2O ¼ 2Hþ þ 2NO�3 ð21Þ

O2ðlÞ þ 2HNO2ðlÞ ¼ 2Hþ þ 2NO�3 ð22Þ

Take all reactions occurred in the absorption process intoconsideration, it can be seen that 2 electrons and 1 electronwere needed for SO2 and NO2 oxidized into SO2�

4 and NO�3 ,respectively. Therefore, when the SO2 and NO2 in flue gas weretotally oxidized, the total amount of electrons needed shouldbe (2[SO2]G-in + [NO2]G-in)QG. Correspondingly, as oxidant, the to-tal electrons supplied should be 2[MnO2] and 4[O2], because twoelectrons and four electrons were supplied when MnO2 and O2

were consumed.Based on the behaviors of SO2 and NO2 in absorption process,

SO2 and NO2 in inlet flue gas should be divided into three sections:(1) oxidized into MnSO4 and Mn(NO3)2 by MnO2; (2) oxidized intoH2SO4 and HNO3 by O2; 3) unabsorbed and discharged in outletflue gas. Due to the electronic conservation in absorption process,the percentage of SO2/NO2 oxidized or discharged could be pre-sented as:

Percentage of oxidized by MnO2

¼ 2d½MnO2�Lð2½SO2�G-in þ ½NO2�G-inÞQ G

� 100% ð23Þ

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Percentage of oxidized by O2 ¼4d½O2�L

ð2½SO2�G-in þ ½NO2�G-inÞQ G� 100%

ð24Þ

Table 1Compositions of pyrolusite (wt%).

MnO2 Fe Ca K Mg Pb Ni Co

27.16 3.44 3.46 1.70 0.58 0.10 0.032 0.017

Fig. 1. Redox potential of MnO2 and existing forms of S(IV) and N(III) with pH.

Fig. 2. Experimental apparatus and flow.

Percentage of discharged in outlet gas

¼ ð2½SO2�G-out þ ½NO2�G-outÞQG

ð2½SO2�G-in þ ½NO2�G-inÞQ G� 100% ð25Þ

where d[MnO2]L is the consumed rate of MnO2, mol/(L s); d[O2]L isthe consumed rate of O2, mol/(L s); [SO2]G-in is the concentration ofSO2 in inlet flue gas, %; [NO2]G-in is the concentration of NO2 in inletflue gas, %; [SO2]G-out is the concentration of SO2 in outlet flue gas, %;[NO2]G-out is the concentration of NO2 in outlet flue gas, %; QG is theflow rate of flue gas, L/s.

Due to the fact that the catalyzed oxidation between O2 andSO2/NOx was so fast and dissolution of O2 into liquid phase was li-quid film controlled mass transfer [42], an assumption was madein order to simply the calculation that the dissolved O2 can be totalconsumed immediately and the concentration of O2 in liquid phaseshould be zero. The consumed rate of O2 in liquid phase shouldequal to its transfer rate between gas and liquid phase.

d½O2�L ¼ KGAG½O2�G-in ð26Þ

where KG is the gas phase mass transfer coefficients, mol/105

(cm2 s Pa); AG is the interfacial area per unit volume, cm�1;[O2]G-in is the concentration of O2 in inlet flue gas, %.

Put Eqs. (6)–(9) together, the balance of SO2 and NOx in absorp-tion process could be presented as:

d½MnO2�Lð½SO2�G-in þ 0:5½NO2�G-inÞQG

þ 2KGAG

Q G

½O2�G�in

ð½SO2�G-in þ 0:5½NO2�G-inÞ

þ ½SO2�G-out þ 0:5½NO2�G-out

½SO2�G-in þ 0:5½NO2�G-in¼ 100% ð27Þ

In a jet bubbling reactor with a specific structure, KGAG can becorrelated to operating parameters, including gas flow rate, con-centration of pyrolusite and agitation speed, when these operatingparameters remained unchanged, the KGAG can be considered asinvariable [43,44]. Therefore, 2KGAG/QG may be considered to beconstant.

Eq. (10) showed that the molar ratio of O2/(SO2 + 0.5NOx) in in-let flue gas was a determined parameter for catalytic oxidation inabsorption process. The catalyzed oxidation would be enhancedwith the increasing O2/(SO2 + 0.5NOx). In the condition of reactionrate between MnO2 and SO2/NOx remained constant, the enhancedcatalyzed oxidation led to an increase in SO2/NOx absorption effi-ciency. On the other hand, if SO2/NOx absorption efficiency re-mained constant, the enhanced catalyzed oxidation led to adecrease in Mn extraction rate.

In the case of O2 being oxidant, H2SO4 and HNO3 were obtainedand lead to the decreasing pH. Variations of redox potential ofMnO2, existing forms of S(IV) and N(III) with pH were displayedin Fig. 1 based on thermodynamic data from data from [45].Fig. 1 showed that the decreasing pH would affect the absorptionprocess from two aspects positively or negatively. On one hand,the redox potential of MnO2 increased with the decreasing pH,enhancing reaction rate between MnO2 and SO2/NOx, which wasgood for Mn extraction rate and SO2/NOx absorption efficiency.On the other hand, as for SO2, the SO2 solubility was reduced be-cause SO2 mainly existed in the form of SO2(aq) and the ionizationsinto HSO�3 and SO2�

3 were inhibited at lower pH, which was bad forSO2 removal. As for NO2, although its solubility was independent ofpH [46,47], lower pH cased the increase of the proportion of HNO2

to total N(III) and accelerated its decomposition with the release ofNO, which was bad for NOx removal.

3. Experiments

3.1. The experimental system

The experimental system was composed of four parts, i.e.,experimental materials, ozone oxidization unit, flue gas treatmentunit and analysis methods. A schematic diagram of the experimen-tal set-up is shown in Fig. 2.

3.1.1. Experimental materialsThe system mainly consists of simulated flue gas supply and

pyrolusite slurry preparation. The simulated flue gas was obtainedby the control of mixing different standard gases using gas flowmeters and the standard gases include O2, SO2, NO and N2 whosepurity were all over 99.9%. Ozone was carried out in an ozone gen-erator with oxygen source (DX-SS1, Harbin jiujiu ElectrochemicalCo., Ltd., China), in which output ozone concentration was 0–10 mg/L, and five gears were prepared to change the outlet ozoneconcentration according to experiment requirement. The pyrolu-site slurry (40 g/L) was prepared by 5 L mixing tap water with200 g commercial pyrolusite from Guangxi Province, China. Theaverage diameter of pyrolusite is 12.015 lm, the compositionsof pyrolusite are shown in Table 1.

Table 2Constant experimental parameters for experiments.

Experiment parameter Value

Flow rate of gas (L/min) 15Pyrolusite concentration (g/L) 40Volume of pyrolusite slurry (L) 5Temperature (�C) 25Agitator speed (r/min) 300

W.-y. Sun et al. / Chemical Engineering Journal 228 (2013) 700–707 703

3.1.2. Ozone oxidization unitThe oxidizations of NO and SO2 in flue gas using ozone is taken

place in an ozonizing chamber, which was a cylindrical glass tubewith inner diameter and length of 5 and 25 cm. Effective volume ofozonizing chamber was calculated to be 500 cm3 with the resi-dence time of 2 s.

3.1.3. Flue gas treatment unitThe absorption of NO2 and SO2 with pyrolusite slurry is taken in

a bubbling reactor, which is a well-stirred sealed vessel (ID,18.5 cm; height, 38 cm) with internal volume of 10 L. Continuousstirring was provided by a mechanical agitator (four blades disc,turbine type impeller) with a speed of 300 rpm/min. The pH valueswere detected by METTLER TOLEDO pH combination electrode(405-DPAS -SC-K8S/325). Temperature was proved by an electricalheater at the bottom of reactor and detected by temperature elec-trode (T817-A-3, Shanghai Precision and Scientific Instrument Co.,Ltd., China). There was also an intelligent PID (Proportion Integra-tion Differentiation) temperature control system which can makesure the error range of temperature to be within 1 �C.

3.1.4. Analysis methodsNegative ions (NO�2 , NO�3 , SO2�

4 and S2O2�6 ) in reactor were ana-

lyzed by ion chromatograph (Dionex-ICS-2500). Concentrations ofSO2, O2, NO, NO2 and total NOx in inlet and outlet flue gas were de-tected by a set of online gas analyzer (SMC-9021, SICK MAIHAKCO., Ltd., Beijing, China). Concentrations of manganese in both li-quid samples and pyrolusite were determined by atomic absorp-tion spectrometer (AA7000 Institute of Eastwest ElectronicTechnology of Beijing, China). Ozone concentration was measuredby the iodometric procedure (Ozone Standards Committee Meth-od), in which ozone was absorbed by neutral solution of potassiumiodine and then the solution was acidified and the liberated iodinewas drawn of by the sodium thiosulfate [48].

3.2. Experiment parameters

In gas-phase oxidation process of NO, ozone was continuouslyinjected into flue gas to oxidized NO into NO2. In absorption pro-cess, SO2 and NO2 were absorbed by pyrolusite slurry undersemi-batch condition. This paper focused on the effect of flue gascomponents on absorption process of SO2 and NOx with pyrolusiteslurry in liquid phase. Inlet concentrations of SO2, NOx and O2 werechanged in order to examine their effects on NOx/SO2 absorptionefficiency and Mn extraction rate. The constant experimentalparameters for the whole experiment were displayed in Table 2.

Fig. 3. Catalytic oxidation reaction in SO2 and NOx removal process with pyrolusiteslurry (SO2 concentration, 2000 ppm; NO concentration, 750 ppm; O2 concentra-tion, 4%; O3 injection, 900 ppm; pyrolusite, 40 g/L). (a) Changes of pH and Mnextraction rate with reaction time. (b) Concentrations of reaction products in liquidphase.

4. Results and discussion

In pro-oxidization process, molar ratio of injected ozone to NOwas controlled at around 1.2:1. In oxidized flue gas, more that96% of NO can be oxidized, among which more than 93% was oxi-dized into NO2 and less than 3% may be oxidized into NO3 andN2O5. The concentrations of NO3 and N2O5 were not detected

because of their low concentrations and lack of on-line detector.Under all conditions, the oxidation rates of SO2 were less than 5%.

4.1. Effect of Mn2+ on oxidation of SO2 and NOx by O2 in the absorptionprocess

Our previous studies have confirmed that SO2 and NOx were to-tally oxidized into SO2�

4 and NO�3 by MnO2 and O2 in absorptionprocess [41], the total amount of SO2 and NOx removed from fluegas should equal to the total concentrations of SO2�

4 and NO�3 in li-quid phase. Under the condition of MnO2 acting as oxidant, Mn2+

was produced as reduction product and products of MnSO4 andMn(NO3)2 were obtained. Based on the conservation of electriccharge, the NOx/SO2 oxidized by MnO2 or O2 could be presented as:

PS=N�MnO2 ¼½Mn2þ�

½SO2�4 � þ 0:5½NO�3 �

ð28Þ

PS=N�O2 ¼½SO2�

4 � þ 0:5½NO�3 � � ½Mn2þ�½SO2�

4 � þ 0:5½NO�3 �ð29Þ

where PS/N-MnO2 is the NOx/SO2 removed by MnO2, %; PS/N-O2 is theNOx/SO2 removed by O2, %; [Mn2+] is the concentration of Mn2+ inliquid phase, mol/L; [SO2�

4 ] is the concentration of SO2�4 in liquid

phase, mol/L; [NO�3 ] is the concentration of NO�3 in liquid phase,mol/L.

Simultaneous absorption of NOx and SO2 with pyrolusite slurrywas taken under semi-batch condition. Variations of reaction pH,

704 W.-y. Sun et al. / Chemical Engineering Journal 228 (2013) 700–707

Mn extraction rate and concentrations of reaction products withreaction time were displayed in Fig. 3.

Results collected in Fig. 3a indicated that SO2/NO2 were oxi-dized by redox reaction between MnO2 and SO2/NO2 and catalyzedoxidation between O2 and SO2/NO2, reaction pH decreased and Mnextraction rate increased gradually with reaction time. Fig. 3bshowed that the concentrations of Mn2+, SO2�

4 and NO�3 almost lin-early increased with reaction time, while the [Mn2+]/([SO2�

4 ] + 0.5[NO�3 ]) remained stable at around 50%, it could beconcluded that the oxidations of SO2/NOx by MnO2 and O2 in theabsorption process should be completely independent.

Fig. 4. Variations of SO2 and NOx absorption efficiency with reaction time underdifferent Mn2+ concentrations (SO2 concentration, 2000 ppm; NO concentration,750 ppm; O2 concentration, 4%; O3 injection, 900 ppm). (a) Variations of NOconcentration in outlet flue gas with reaction time under different Mn2+ concen-trations. (b) Variations of NOx absorption efficiency with reaction time underdifferent Mn2+ concentrations. (c) Variations of SO2 absorption efficiency withreaction time under different Mn2+ concentrations.

It must be pointed out that the increasing Mn extraction rate in-creases the Mn2+ concentration, which may act as catalyst for theoxidizations of SO2 [49–53] and NOx by O2, this may have a consid-erable effect on oxidization rate. Some experiments were carriedout to investigate the effect of concentrations of Mn2+ on the oxi-dation of SO2 and NOx by O2 in the absence of pyrolusite. Resultscollected in Fig. 4 showed that the oxidations of SO2 and NO2 byO2 were enhanced in the presence of Mn2+, lower the concentra-tions of SO2, NO2 and HNO2 in liquid phase and, as a result, thedecomposition of HNO2 into NO was restrained (Fig. 4a) and boththe NOx and SO2 efficiencies were improved (Fig. 4b and c). Theseimprovements both on NOx and SO2 efficiencies were proportionalto the increase of Mn2+ concentration from 0 to 0.01 mol/L, but notchanged obviously when Mn2+ concentration exceeded 0.01 mol/L.Similar results have also been reported by Berglund [50] in themanganese-catalyzed oxidation of SO2 by oxygen in aqueous solu-tion and the reason may be the change of reaction order of Mn2+

from 1 to 0 with the Mn2+ concentration increasing [50].When pyrolusite was used as absorbent, the Mn extraction rate

reached 14% (Mn concentration of 0.018 mol/L) in the initial 2 h(Fig. 3a), while the further increase of Mn concentration has negli-gible impact on the oxidization rates in the next 12 h. That is thevariations of Mn concentration have little influence on SO2/NOx

oxidization rates in our experiments, that may be the seasonwhy the ratio of [Mn2+]/([SO2�

4 ] + 0.5[NO�3 ]) in liquid phase didnot change with reaction time, but remained stable at around 50%.

SO2 and NOx in liquid phase were oxidized by both of MnO2 andO2. With the gradual reduction of MnO2 in the liquid phase, thesystem oxidation would be diminished. Furthermore, H2SO4 andHNO3 produced by catalyzed oxidation led to lower reaction pH,leading to the low solubility of SO2 and the decomposition ofHNO2 in liquid phase. Both the two factors led to the decrease ofSO2 and NOx absorption efficiencies. Therefore, in order to obtainsatisfying NOx absorption efficiency with high SO2 absorption effi-ciency at the same time, we set the SO2 absorption efficiency at90% as the end of semi-batch reaction. The concentrations of O2,SO2 and NOx in flue gas were changed and the effects of O2/(SO2 + 0.5NOx) on reaction products, Mn extraction rate, NOx/SO2

absorption efficiency and absorption capacity in SO2 and NOx

absorption process were discussed on the base of SO2 absorptionefficiency at 90%.

4.2. Effect of O2/(SO2 + 0.5NOx) on SO2 and NOx absorption

4.2.1. On reaction pH and productsVariations of reaction pH and [Mn2+]/([SO2�

4 ] + 0.5[NO�3 ]) withO2/(SO2 + 0.5NOx) were displayed in Fig. 5.

Fig. 5a showed that both the pH and [Mn2+]/([SO2�4 ] + 0.5[NO�3 ])

all decreased with the increasing O2/(SO2 + 0.5NOx) no matter thechange of O2/(SO2 + 0.5NOx) was caused by O2, SO2 or NOx. It canbe explained by that the increasing O2/(SO2 + 0.5NOx) led to theincreasing proportion of SO2 and NOx removed by catalyzed oxida-tion, and more H2SO4 and HNO3 were produced.

Fig. 5b showed that in the condition of O2/(SO2 + 0.5NOx) < 18,more than 50% of SO2/NOx was oxidized by MnO2 with the mainproducts of MnSO4 and Mn(NO3)2, while when O2/(SO2 + 0.5-NOx) > 18, SO2/NOx was mainly oxidized by O2 with the main prod-ucts of H2SO4 and HNO3. When O2/(SO2 + 0.5NOx) at 40, [Mn2+]/([SO2�

4 ] + 0.5[NO�3 ]) fell to 13%, showing that more than 87% ofNOx and SO2 were oxidized by O2.

4.2.2. On NOx absorption efficiencyVariations of NOx absorption efficiency with O2/(SO2 + 0.5NOx)

were displayed in Fig. 6, which showed the NOx absorption effi-ciency NOx was not effected with the O2/(SO2 + 0.5NOx) caused

Fig. 7. Effect of O2/(SO2 + 0.5NOx) ratio on SO2/NOx absorption capacity. (a) Effect ofO2/(SO2+0.5NOx) ratio on reaction time (b) Effect of O2/(SO2 + 0.5NOx) ratio on totalabsorption capacity of SO2 and NOx.

Fig. 8. O2/(SO2 + 0.5NOx) ratio dependence of Mn extraction rate.

Fig. 5. Reaction pH and reaction products at different O2/(SO2 + 0.5NOx) ratio. (a)Reaction pH at different O2/(SO2 + 0.5NOx) ratio. (b) Reaction products at differentO2/(SO2 + 0.5NOx) ratio.

Fig. 6. NOx removal efficiency as a function of O2/(SO2 + 0.5NOx) ratio.

W.-y. Sun et al. / Chemical Engineering Journal 228 (2013) 700–707 705

by O2 and SO2, while decreased with the increasing O2/(SO2

+ 0.5NOx) caused by NOx. That is, NOx absorption efficiency wasindependent of O2 and SO2 but increased with increasing inletNOx concentration. The reason might be that in a wet NO2 absorp-tion process, the gas–liquid mass transfer rate of NOx was propor-tional to the concentration driving force according to film theoryand the increase of gas partial pressure increased the concentrationdriving force between gas phase and liquid phase [9,43]. As a re-sult, the gas–liquid mass transfer rate of NOx was enhanced withthe increasing inlet NOx concentration.

4.2.3. On NOx/SO2 absorption capacityThe effects of O2/(SO2 + 0.5NOx) on reaction time and NOx

absorption capacity were displayed in Fig. 7. Fig. 7 showed thatthe increasing O2/(SO2 + 0.5NOx) prolonged the reaction time andNOx/SO2 absorption capacity. It could be explained by that in thesemi-batch condition, when the same pyrolusite was added inthe experiments, the increasing O2/(SO2 + 0.5NOx) meant theincreasing amount of oxidant, which promoted the consumed rateof NOx/SO2, led to the lower concentrations of NOx/SO2 in liquidphase and increased the concentration driving force between gas

Table 3Effect of rhodochrosite addition on Mn extraction rate and total absorption capacity of SO2 and NOx.

Absorbent pH Mn extraction rate (%) SO2�4 (mol L�1) NO�3 (mol L�1) Reaction time (h)

Pyrolusitea 0.48 59.00 0.253 0.094 15.73Pyrolusite and rhodochrositeb 0.90 94.87 0.386 0.145 23.86

a Mn content of pyrolusite, 17.17%, concentration of pyrolusite, 40 g/L.b Mn content of pyrolusite, 17.17%, concentration of pyrolusite, 40 g/L; Mn content of rhodochrosite, 23.10%, concentration of pyrolusite, 60 g/L.

706 W.-y. Sun et al. / Chemical Engineering Journal 228 (2013) 700–707

phase and liquid phase. As a result, the reaction time was pro-longed and NOx/SO2 absorption capacity was enhanced.

4.2.4. On Mn extraction rateVariations of Mn extraction rate with O2/(SO2 + 0.5NOx) were

displayed in Fig. 8. Fig. 8 showed that Mn extraction rate increasedfirst and then dropped with the rise of O2/(SO2 + 0.5NOx), the high-est Mn extraction rate of 91% was achieved when O2/(SO2 + 0.5-NOx) at around 13. It could be explained from three aspectspositively or negatively. Firstly, the increase of the O2/(SO2 + 0.5-NOx) promoted the reaction between O2 and SO2/NOx and reducedthe reaction rate between MnO2 and SO2/NOx, leading to the de-crease in Mn extraction rate. Secondly, H2SO4 and HNO3 were pro-duced and led to lower pH, while the redox potential of MnO2

increased with the decreasing pH, enhanced reaction rate betweenMnO2 and NOx/SO2, which was good for Mn extraction. Thirdly, thelower pH led to the low solubility of SO2 and the decomposition ofHNO2 in liquid phase, which also contributed to the decreasingSO2/NOx removal efficiency and low Mn extraction rate. Whenthe positive effect was greater than the negative effect, the Mnextraction rate increased, otherwise, it decreased.

4.3. Measures for improving Mn extraction rate

For a removal process of SO2 and NOx, the absorption efficiencyand capacity were key parameters. However, for a resource utiliza-tion process of pyrolusite, another key parameter which should beconsidered was the Mn extraction rate. When O2/(SO2 + 0.5NOx) -P 18, SO2 and NOx were mainly oxidized by O2 with main productsof H2SO4 and HNO3, the low pH led to low solubility of SO2 anddecomposition of HNO2, and eventually lead to the low SO2/NOx re-moval efficiency and Mn extraction rate. In order to overcome thisproblem, rhodochrosite, in which Mn existed in the form of MnCO3,was added to neutralize the H2SO4 and HNO3 to increase SO2 solu-bility and inhibit HNO2 decomposition through reaction (30).

MnCO3ðSÞþH2SO4 ! MnSO4þH2Oþ CO2 " ð30Þ

In the condition of O2 at 8%; SO2 at 2000 ppm and NOx at1000 ppm, a contrast test was carried out to research the variationsof absorption process before and after adding rhodochrosite.Table 3 showed that the addition of rhodochrosite caused the pHincreased from 0.48 to 0.90, the reaction time increased from15.73 h to 23.86 h, and eventually the Mn extraction rateand SO2/NOx absorption capability increased from 59.00%,0.253 mol/L and 0.094 mol/L to 94.87%, 0.386 mol/L and0.145 mol/L, respectively. The addition of rhodochrosite achievedthe aims of improving SO2/NOx absorption capability and makingfull use of manganese resources at the same time.

5. Conclusions

This paper focused on the effect of flue gas components onabsorption process of SO2 and NOx with pyrolusite slurry. The O2/(SO2 + 0.5NOx) (molar ratio) in the flue gas was put forward as acorrelated parameter, and its effect on catalyzed oxidation,byproducts, Mn extraction rate, SO2/NOx absorption efficiency

and absorption capacity, were investigated. The following specificconclusions can be drawn from the experimental results.

(1) Increasing O2/(SO2 + 0.5NOx) enhanced the catalyzed oxida-tion between O2 and SO2/NO2 in liquid phase. When O2/(SO2 + 0.5NOx) 6 18, SO2 and NOx were mainly oxidized byMnO2 with main products of MnSO4 and Mn(NO3)2, whilewhen O2/(SO2 + 0.5NOx) P 18, SO2 and NOx were mainly oxi-dized by O2 with main products of H2SO4 and HNO3.

(2) In the respect of SO2 and NOx removal, NOx absorption effi-ciency was not affected by concentrations of O2 and SO2,while increased with increasing NOx concentration. The totalabsorption capability of NOx and SO2 increased with increas-ing O2/(SO2 + 0.5NOx).

(3) In the respect of comprehensive utilization of pyrolusite, Mnextraction rate increase with the increase of O2/(SO2 + 0.5-NOx) at first and then decrease, the maximum Mn extractionrate of 91% was got when O2/(SO2 + 0.5NOx) at around 13.

(4) For flue gas of O2/(SO2 + 0.5NOx) P 18, both the Mn extrac-tion rate and SO2/NOx absorption capability could beenhanced by adjusting the pH value of the system via addingrhodochrosite in it.

(5) The study provided the theoretical basis for obtaining satis-fied SO2/NOx absorption capability and Mn extraction ratefor flue gas with different components.

Acknowledgements

This project is supported by Program for New Century ExcellentTalents in University (NCET-09-0570), Special Foundation forYoung Scientists of Sichuan Province (2011JQ0008), China Postdoc-toral Science Foundation funded project (2013M531966) and Post-doctoral Science Foundation funded project of Sichuan Province.

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